The storage/release capability and chemical composition of synaptic vesicles will be compared in resting, moderately activated, and intensively activated electromotor nerve terminals of Torpedo in vitro. Stimulation-induced alteration in vesicle morphology and its reversal during subsequent rest will be studied on the ultrastructure level by stereological analyses and by application of exogenous, high molecular weight tracers (dextrans, horseradish peroxidase). The releasable acetylcholine (ACh) pool in intact resting and stimulated terminals will be compared with the amounts of vesicular ACh isolated from these terminals; the recovery of the releasable pool and isolatable vesicular ACh after stimulation will be followed using radioactive precursors of ACh (acetate or choline). Concomitantly, miniature end-plate potentials will be determined in order to learn how the frequency and amplitude of MEPPs vary as a function of the alteration of transmitter stores in terminals. Vesicles isolated from resting and stimulated tissue will be purified, their protein composition will be analyzed by sodium dodecylsulfate polyacrylamide slab gel electrophoresis, and their lipid composition will be determined. Combined ultrastructural, electrophysiological, and biochemical results will be used to define how the cycling of vesicles affects their storage/release capability and hence the functional state of presynaptic terminals.